The present invention relates generally to exposure systems used in illuminating original documents for copying and, more particularly, to such a system wherein the original is scanned by a slit of exposure light. The invention provides a means for regulating contrast in the reproduction image produced using the exposure system.
A number of copying devices are known wherein an original document is illuminated and the illumination is directed onto a photosensitive media to produce an image. A variety of photosensitive surfaces may be used, such as a charged photosensitive drum in the case of conventional xerography. As further examples, other media may be used such as conventional photographic film or a micro-encapsulated photosensitive media such as that disclosed in U.S. Pat. Nos. 4,440,846 and 4,399,209.
A common method of illuminating the original document is by scanning the document with a slit of exposure light. In a typical prior art arrangement. a linear light source positioned within a parabolic cylinder reflector directs a slit of light onto the plane within which the original is held. The light is reflected from the original and collected by a lens or other focusing means, whereafter it is directed onto the photosensitive media. The light source and focusing system may be moved together past the original to scan the document, or the original may be moved past the light source and focusing system.
In such a copying device, it is advantageous to be able to control the contrast level in the output image. While high contrast is desirable in images comprising text or line drawings, such high contrast results in low quality reproductions when copying continuous tone originals. Since a particular copying apparatus may be expected to reproduce originals of either type, control of contrast would greatly enhance the capabilities of the apparatus.
It is known that contrast can be reduced through the use of a "bump" exposure. Such an exposure is an overall, nonimage exposure of the photosensitive media. This exposure has the effect of reducing or softening the contrast of the image once the exposure is made. The bump exposure can be carried out prior to, after or simultaneous with the image exposure.
It is desirable to be able to control the extent of the bump exposure so as to control contrast. With a fixed bump exposure, the reproduction image can be unacceptably low in density in areas of the image such as text or line graphics. A controlled bump can be provided using means such as an auxiliary bump light source, or by providing separate bump and image exposures using the same light source. These techniques have disadvantages, however, in that the required control systems add cost and complexity to the copying apparatus. In the case of an auxiliary lamp, further cost and complexity is added through the additional light source. Where separate bump and image exposures are made, the time required to produce an image is increased. In either case, the power requirements of the apparatus are increased.
What is needed, therefore, is a convenient means by which to provide a controlled bump exposure for contrast control in a reproduction image. Such a means should be easily controllable, and yet add relatively little to the cost and complexity of the reproduction apparatus.